1. Field of the Invention
The present invention relates to new di-arylmethoxy-dinitro-benzenes, a process for their preparation from dihalogeno-dinitro-benzenes and arylmethanols, as well as their further conversion to 4,6-diaminoresorcinol by catalytic hydrogenation.
Diaminoresorcinol is used as a monomer unit in the synthesis of polybenzoxazoles, which, for example, are used as starting material for electroactive polymers (DE-OS 3,341,627).
2. Description of the Related Art
At present diaminoresorcinol is prepared by nitration of diacetylresorcinol, removal of the protective groups and reduction of the nitro groups on a Pd/activated charcoal catalyst (Macromolecules 14 (1981), 909). The critical step in this process is the nitration. In nitric acid is proceeds ony with approximately 30% yield. Since, the process must also be carried out at high dilution, the space yield is only about 10 g/l or 0.05 mole/l. The removal of the protective group in a further process step is carried out with similarly unfavourable space yield. Styphnic acid (2,4,6-trinitroresorcinol) is formed as by-product, which leads to deflagrations at elevated temperatures (Bielstein, Main Work volume 6, p. 830) and therefore constitutes a safety problem. Since, on the one hand, the reaction is strongly exothermic and, on the other hand, exothermic decomposition reactions occur at a temperature which is only slightly above room temperature, a safe reaction procedure is difficult.
According to U.S. Pat. No. 4,745,232, the formation of dinitroresorcinol from diacetylresorcinol is said to proceed without the undesired formation of styphnic acid and with a 65% yield if the process is carried out using "white" nitric acid (presumably HNO.sub.3 which is free from NO.sub.2 is intended here) and with addition of urea as a reagent for the control of the nitrosyl ions, or if the process is carried out with nitric acid in sulphuric acid. With these procedures the space yield improves to 65 g/l (0.35 mole/l) and to 40 g/l (0.2 mole/l) respectively but, for this improvement the increased expense mentioned is necessary.
Because of the difficulties mentioned it has been tried to obtain diaminoresorcinol from another starting material. Thus, in EP 266,222 it is proposed to nitrate 1,2,3-trichlorobenzene in a sulphuric acid/nitric acid mixture to 1,2,3-trichloro-4,6-dinitro-benzene and to convert this compound, using an alkali metal hydroxide in methanol/water, into 1,3-dihydroxy-2-chloro-4,6-dinitrobenzene. The reduction of the latter compound on a Pd/activated charcoal catalyst gives diaminoresorcinol. However this process has the disadvantage that the dihydroxy compound of the second process step must be extracted with ethyl acetate, as a consequence of which the space yield drops to about 75 g/l (0.32 mole/l); after the said extraction the dihydroxy compound is isolated by evaporation of the ethyl acetate phase.